Investigations of spin currents in elemental, alloyed and multilayer systems using spin-torque ferromagnetic resonance technique

• Main Author: Coester, Birte
• Other Authors: Lew Wen Siang
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Science::Physics
• Online Access: https://hdl.handle.net/10356/170477
• Institution: Nanyang Technological University

Current-induced spin-orbit torques (SOT) are a vital research field within the realm of spintronics. They have attracted significant attention due to the vast opportunities for applications, e.g., in neuromorphic computing or memory devices, and their potential to reduce energy consumption in said applications. Energy efficient generation of SOT is an enormous part of today’s spintronics research, and a simple yet powerful way to generate them is the spin Hall effect (SHE), which converts charge currents into spin currents due to the spin-orbit coupling induced spin-dependent scattering in a nonmagnetic material (NM). When the spin currents accumulate at the interface to an adjacent ferromagnet (FM), they can exert a fieldlike SOT, and when they are absorbed into the FM, they exert a damping-like SOT onto the magnetization. It is thus essential to examine the spin-charge conversion ratio, which can be achieved by determining the ferromagnetic resonance (FMR) efficiency with spin-torque ferromagnetic resonance (STFMR) measurements, which are based on measuring the rectification of an alternating current and oscillating resistance due to two magneto-resistive effects, namely, the anisotropic magnetoresistance (AMR) and the spin Hall magnetoresistance (SMR). Further, it allows simultaneous characterization of the damping parameter and, by applying an additional bias current, the individual SOT efficiencies. Achieving a high spin-charge conversion ratio is highly desired as it can improve SOT efficiencies and reduce energy consumption in applications. One method to enhance the spincharge conversion ratio is to increase the scattering cross-section by introducing impurities into the NM. However, this results in an increase in resistivity and, thus, a drop in spin Hall conductivity (SHC), which is not advantageous regarding energy efficiency. Therefore, I studied the FMR efficiency and the SHC in a CuW alloy and its effect on the damping in an adjacent Fe layer. I found that at a W concentration of 60%, the spin-charge conversion ratio and the SHC are enhanced. Additionally, the ratio of the FMR efficiency to damping, which is proportional to the charge current density needed to switch an in-plane magnetization, is decreased by a factor of 4 compared to a pure W layer. I further investigate the spin-charge conversion enhancement using a multilayer structure of alternating W and Cu layers on a Fe layer. By increasing the number of interfaces in the NM, a significant enhancement up to a factor of 84 was observed, compared to a single layer of W given the same thickness. Further, it was shown that while the Cu layers did not much affect the spin current in the NM, a spin accumulation arose in the W layers, explaining a rather large field-like SOT and a reduction in damping-like SOT. Finally, out-of-plane STFMR measurements were performed on a conventional Co/Pt heterostructure to entangle the contributions of AMR and SMR. These measurements revealed the potential to control the spin current that crosses the FM/NM interface and the SOT by frequency and bias current dependent STFMR measurements in the out-of-plane direction.